Hi Vishal,
Its a OSPF design/behaviour that all areas need to be connected to OSPF backbone area(i.e Area0).
If in case the area is not connected to area0 there are two methods which can be used and they are (Virtual Link & Tunnel). But we recommend this to be used only for temp solution.
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094aaa.shtml#areas
An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification.. A router within an area must maintain a topological database for the area to which it belongs. The router doesn't have detailed information about network topology outside of its area, thereby reducing the size of its database.
Areas limit the scope of route information distribution. It is not possible to do route update filtering within an area. The link-state database (LSDB) of routers within the same area must be synchronized and be exactly the same; however, route summarization and filtering is possible between different areas. The main benefit of creating areas is a reduction in the number of routes to propagate—by the filtering and the summarization of routes.
Each OSPF network that is divided into different areas must follow these rules:
- A backbone area—which combines a set of independent areas into a single domain—must exist.
- Each non-backbone area must be directly connected to the backbone area (though this connection might be a simple logical connection through a virtual link, ).
- The backbone area must not be partitioned—divided into smaller pieces—under any failure conditions, such as link or router down events.
Please find below link which explain the behaviour of Virtual Links and Areas:-
All areas in an Open Shortest Path First (OSPF) autonomous system must be physically connected to the backbone area (Area 0). In some cases, where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. You can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area. The area through which you configure the virtual link, known as a
transit area
, must have full routing information. The transit area cannot be a stub area. This document examines the OSPF database in a virtual link environment.
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ee.shtml
HTH
Regards
Inayath
*Plz rate if this info is helpfull
